A recent question was received via e-mail…
We are encountering cracking at the bosses on a polysulfone part. We press fit inserts on the bosses but we don’t see cracking during insertion. It is not until a day or two later that we see the cracking. Any thoughts?
The occurrence of cracking and crazing in this case is most likely due to the factors of molded-in stresses and strain.
Since polysulfone has a high degree of semi-crystallinity and rigidity, the resulting parts have many molded-in stresses… especially in the case of a boss. Many injection molders will anneal the parts with a heat cycle to reduce these molded-in stresses. Doing this before and after the insertion would significantly reduce these stresses.
Reducing the strain during insertion will also reduce the likelihood of cracking and crazing. Using a barbed, threaded, or trilobal insert may reduce this strain. The use of ultrasonic insertion or in-mold insertion will also reduce this.
It may also be a good idea to mold the part in a translucent material so that you can see where the stresses are present under polarized light.
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